1. Field of the Invention
The present invention concerns an x-ray apparatus having a radiation source operated by a voltage generator, a digital solid-state radiation detector, and a control device controlling the operation of the apparatus.
2. Description of the Prior Art
In recent years, digital x-ray detectors have changed classical radiography, fluoroscopy, angiography and cardio-angiography. Among others, image intensifier camera systems based on television or CCD cameras, storage film systems with integrated or external readout units, systems with optical coupling of the converter foil to CCDs or CMOS chips, selenium-based detectors with electrostatic readout and solid-state detectors with active readout matrices with direct or indirect conversion of the x-ray radiation, represent such digital technologies.
In particular, solid-state detectors play a large role. Such detectors are based on active readout matrices, for example made of amorphous silicon (a-Si). In an x-ray converter (transducer), for example cesium iodide (CsI), the image information is initially converted into light and is subsequently converted into electrical charge in the photodiodes of the matrix and stored there. Related technologies likewise use an active readout matrix made from amorphous silicon, but employ a transducer that generates direct electrical charge (for example, selenium), this charge being subsequently stored on an electrode. In each case, the stored charge is subsequently read out by an active switching element with dedicated electronics and is converted from analog-to-digital form and further processed by the image system.
Planar image detectors based on amorphous materials or semiconductor materials (for example, planar image detectors with indirect conversion by means of a scintillator and a matrix made from amorphous silicon, or detectors with directly-converting materials such as selenium, lead oxide, lead iodide, cadmium telluride) possess physical properties that can lead to ghost image artifacts. Ghost images mean images that contain a persisting signal of an earlier exposure as well as the current signal (an x-ray signal of a patient or another subject, or a dark signal). Among other things, this persisting signal portion significantly adulterates the actual image signal and can lead to a false interpretation of the current image. It is therefore desired to reduce the ghost image signal to a level that can no longer by perceived, and therefore that no longer interferes.
There are in principle a number of possibilities for attempting such a result. A first possibility is the use of a reset light as, for example, is used in planar image detectors made from amorphous silicon. The detector matrix is thereby completely illuminated with light for a short time, and thus the individual charges stored pixel-by-pixel are raised to a uniform level. However, this does not lead to a complete reduction of the artifact. A further possibility is the use of suitable software methods to reduce the artifacts; meaning that they are mathematically “calculated out” in the downstream image processing. In practice, however such model computation has proven to be unsuitable.